1. Field of the Invention
This invention relates to electron guns and more particularly to measurement of current leakage and providing a response to current leakage in electron guns.
2. Description of Related Art
Electron beam lithography systems typically operate at acceleration voltages of 10-100 kV. Leakage current, which can be caused by dirty high voltage insulators, is detrimental for the following reasons:
(1) excessive leakage leads to beam current instability;
(2) excessive leakage at high voltages can lead to high values of x-ray emission out of the gun; and
(3) excessive leakage can lead to an arc which can damage static-sensitive components.
To date, no method exists for measuring leakage current in real-time.
U.S. Pat. No. 3,885,194 of Schumacher for xe2x80x9cTemperature Control for an Indirectly Heated Cathode for a High Power Electron Beam Gunxe2x80x9d shows an electron beam welding system with an electron gun with a bolt cathode surrounded by an auxiliary cathode in the form of a tungsten filament which generates electrons which bombard the cathode. There is a filament power supply connected to provide excitation and heating of the auxiliary cathode. There is a voltage supply which energizes the filament power supply. The voltage supply produces a voltage which varies in response to feedback in the form of a control signal from a sensing member which may be a current measuring device which is employed to estimate the cathode temperature as a function of current therethrough. There is a variable potential source which is connected to energize a circuit which includes an annular auxiliary anode, the cathode and the sensing member.
U.S. Pat. No. 4,000,440 of Hall et al. for xe2x80x9cMethod and Apparatus for Controlling Brightness and Alignment of a Beam of Charged Particlesxe2x80x9d shows an E-beam system with a electron gun which produces a beam of electrons. In that system, it was necessary to assure that the current density of the E-beam would have a uniform current density. To that end it was also necessary to assure that the brightness of the E-beam be maintained near a constant level. As stated by Hall xe2x80x9cBecause the brightness depends upon the relationship between the temperature of the cathode and cathode emission, it is necessary to correct for beam brightness of the beam if the beam is to be properly aligned.xe2x80x9d That was because xe2x80x9ca slight change in the temperature of the cathode heater can cause a substantial change in the current density of the beam through substantially changing the total beam current. If the beam does not have the desired total current it . . . cannot be properly aligned.xe2x80x9d
xe2x80x9cIf the beam is not properly aligned, the current for the cathode heater current might be increased until a maximum beam current was produced by the cathode . . .xe2x80x9d to put the beam current in a desired range. Such a condition would endanger a reduction in the life of the cathode. In Hall et al., the brightness level is controlled without having to increase the heater current to obtain a maximum current output from the cathode.
U.S. Pat. No. 4,568,861 of Doran et al. for xe2x80x9cMethod and Apparatus for Controlling Alignment and Brightness of an Electron Beamxe2x80x9d shows a E-beam system with a beam shaping aperture having an output connected to the input of an operational (I-E) amplifier. The output of the I-E amplifier is connected to the input of a voltage-to-frequency converter (V/F) which passes through a NAND gate to its output. The output of the NAND gate is supplied to a series of up-counters so that at the end of one xe2x80x9cdither cyclexe2x80x9d the counters store a count proportional to the total beam current collected by the shaping aperture plate during that dither cycle. The count in the up-counters is transferred by a data bus and a microprocessor to a brightness digital-to-analog (DAC) converter which outputs a D.C. voltage to the filament power supply so that the power to the filament varies as a function of the difference between the output of the DAC and a reference voltage which corresponds to the desired brightness of the beam.
In shaped-beam electron beam lithography machines, the beam is xe2x80x9cservoedxe2x80x9d periodically, to ensure that it is centered in the apertures, and that the beam current measured at some reference point (sample aperture or Faraday cup) is constant. Emission from the electron source is adjusted, or servoed, when needed to ensure that the beam current at this reference point is held constant. A similar beam servo technique will occur on an Electron Bean Projection System (EBPS) column. However, it is expected that the servoing which will occur about once per wafer and will not affect throughput of the tool.
When a leakage current exists in an electron gun, there is a reduction in the efficiency of the current from the high voltage power supply output. For example, suppose that the high voltage unit is set to 1000 xcexcA to produce a target current of 25 xcexcA in a projection electron beam system. The difference between the 1000 xcexcA and the 25 xcexcA target current is due to the beam current being trimmed at a series of apertures in the system. An upper aperture (shaping aperture in an EBPS system) can typically stop at least xc2xd of the beam current incident on it. Any change in current on this upper aperture, with respect to the current that the high voltage unit is producing, is a measure of the leakage current in the electron gun. If a 20 xcexcA current leakage path develops in the electron gun, then the useful current from the high voltage unit is reduced by 20 xcexcA (and in the example above, the target current is reduced by 0.5 xcexcA). The conventional servo, as outlined above, would detect that the current at the target has dropped by 0.5 xcexcA, and raise the emission of the high voltage unit to 1020 xcexcA to compensate for the leakage. The emission current from the high voltage unit is not recorded on present-day systems, so no record of this leakage results. Also, real-time current on the wafer needs to be stable.
Monitoring the current from the high voltage unit, after each servo update, and comparing to a reference current is an effective way of determining the leakage current, but slow, since it is updated only during the servo cycle.
In accordance with this invention, a method and a system are provided for operating an E-beam system including an E-beam source for generating an E-beam. directed along a column axis. Direct the E-beamn towards means for measuring a parameter of the E-beam. Generate a leakage signal representing leakage current emitted from the E-beam, and generate an excess leakage signal when the result of a comparison with a desired value is excessive. The excess leakage signal can be provided as an emergency output signal and/or produce an OFF signal for stopping production of the E-beam by turning OFF voltage/power sources for producing the E-beam in response to the excess leakage signal. Preferably, a filament is heated by an electric current and a cathode is bombarded with electrons from the filament to produce the E-beam. Then a filament control signal is employed for controlling the filament heating current.
Preferably produce the E-beam by heating the filament for emitting electrons proximate to a cathode aligned therewith, and provide a conductive aperture located along the column axis. Provide an adjustable filament emission power for generating an output to accelerate electrons from the filament to bombard the cathode. Supply cathode emission power supply for generating a cathode emission current and voltage to accelerate the E-beam from the cathode along the column axis. Measure the cathode emission current. Provide for the conductive aperture to be adapted to intercept electrons from the cathode, thereby producing an aperture current. Measure the aperture current. Generate a ratio of the aperture current to the cathode emission current. Test to determine whether the ratio is within a limit, and generate an excess leakage output signal when the ratio is outside of the limit. Preferably, generate an emergency output signal when the ratio is outside of the limit; provide a filament control signal for adjusting filament emission power; and provide an, OFF signal for causing turning off power supplied to the E-beam when the ratio is outside of the limit by generating an excess leakage signal.
In accordance with another aspect of the method and system of this invention, provide an electron gun including a filament, a cathode located along the axis below the filament, and a shaping aperture located along the column axis below the cathode. Provide a first voltage source for heating the filament to cause thermionic emission of electrons. Provide a second voltage source for supplying a bombardment accelerating voltage to direct the electrons along the column axis towards the cathode. Provide a third voltage source coupled to the shaping aperture for supplying a beam accelerating voltage to direct an E-beam generally along the column axis, the E-beam comprising energized electrons couple to the beam accelerating voltage. Generate an emission current signal representing the electron source current emitted from the electron source. Generate a shaping aperture current signal representing current flow at the shaping aperture. Compare the shaping aperture current signal and the current emission current signal and provide a comparison ratio output signal. Test to determine whether the comparison ratio output signal is within a limit. Generate an output signal when the comparison output signal is outside the limit.